1. Field of the Invention
The present invention relates to a motor driving apparatus for use in a zoom drive motor of a consumer video camera or the like.
2. Description of the Prior Art
A conventional motor driving apparatus for use in a zoom drive motor of a consumer video camera, for example, is shown in FIG. 1 of the accompanying drawings.
As shown in FIG. 1, a PWM (pulse width modulation) signal generating circuit 41 which derives a PWM signal for driving a motor is provided. The PWM signal generating circuit 41 includes a read-only memory (ROM) 42 in which data of a PWM signal corresponding to a sine wave, for example, is stored. When supplied with a signal representative of an arbitrary rotary phase, the PWM signal generating circuit 41 generates a PWM output signal corresponding to the sine wave.
The PWM output signal from the PWM signal generating circuit 41 is supplied to an EN input of an H-type bridge driver circuit 43. A DIR 1 output signal that corresponds to a motor driving direction is generated from the generating circuit 41. The DIR 1 output signal from the generating circuit 41 is supplied to an IN 1 input of the H-type bridge driver circuit 43.
A and B outputs of the H-type bridge driver circuit 43 are supplied to respective wire ends of a coil 45 of a motor (M) 44. The coil 45 of a the motor 44 is of two-phase type and therefore the PWM output signal and the DIR output signal from the generating circuit 41 and EN, IN inputs and A, B outputs of the H-type bridge driver circuit 43 are each generated in the form of two channels as shown in FIG. 1.
The H-type bridge driver circuit 43 generates the A and B outputs for the EN and IN inputs as shown in the following truth table 1.
TABLE 1 ______________________________________ Case IN1 (1N2) EN1 (EN2) A1 (A2) B1 (B2) ______________________________________ 1 L H L H 2 H H H L 3 L L Hi-Z Hi-Z 4 H L Hi-Z Hi-Z ______________________________________ (Hi-Z: High Impedance)
The above-mentioned truth table 1 will be described in association with operation of a typical circuit. The H-type bridge driver circuit 43 is arranged as shown in FIG. 2, for example. FIG. 2 shows only one phase side of the coil 45 of the motor 44, and the coil 45 is represented by a series circuit 51 of a coil and a resistor.
As shown in FIG. 2, one end A of the series circuit 51 is connected through a first switch S.sub.1 and a diode D.sub.1 to a drive voltage source 52 and is also grounded via a second switch S.sub.2 and a diode D.sub.2. The other end B of the series circuit 51 is connected through a third switch S.sub.3 and a diode D.sub.3 to the drive voltage source 52 and is also grounded via a fourth switch S.sub.4 and a diode D.sub.4. The switches S.sub.1 to S.sub.4 may be replaced with ICs, such as transistors or the like, in actual practice.
In the H-type bridge driver circuit 43, a set of diagonally located switches S.sub.1 and S.sub.4 and a set of diagonally located switches S.sub.2 and S.sub.3 are driven commonly. When the EN input is at high potential (H) and the IN input is at high potential (H), i.e., in the case 2 of the truth table 1, a set of switches S.sub.1 and S.sub.4 are turned on and a set of switches S.sub.2 and S.sub.3 are turned off so that voltages of A1, A2 and B1, B2 go to high (H) level and low (L) level as shown on the truth table 1, causing an electric current to flow from one end A to the other end B. When the EN input is at high potential (H) and the IN input is at low potential (L), i.e., in the case 1 of the truth table 1, a set of the switches S.sub.2 and S.sub.3 are turned on and a set of the switches S.sub.1 and S.sub.4 are turned off so that voltages of the A1, A2 and B1, B2 go to low (L) level and high (H) level as shown on the truth table 1, causing an electric current to flow from the other end B to one end A. Further, when the EN input is at low potential (L), i.e., in the cases 3 and 4 of the truth table 1, the switches S.sub.1 to S.sub.4 are all turned off so that the voltages of A1, A2 B1 and B2 are all set in the high impedance (Hi-Z) state as shown on the truth table 1.
Accordingly, the aforesaid PWM output signal is supplied to the EN input of the H-type bridge driver circuit 43 and the DIR output signal is supplied to the IN input of the H-type bridge driver circuit 43, whereby an average electric current corresponding to the PWM output signal is supplied to the coil 45 of the motor 44 in the direction represented by the DIR output signal, thereby driving the motor 44.
FIG. 3 shows a relationship between a duty ratio of the PWM signal and an average electric current. That is, a characteristic of average current becomes nonlinear in the portion where the duty ratio of the PWM signal is less than 58%. Therefore, it is customary that the position at which the duty ratio is less than 58% is regarded as "0" and the electric current is inverted at this position "0" in the reverse direction to drive the motor.
When the electric current is inverted in the reverse direction at the position where the duty ratio is 58%, then the waveform of a drive signal becomes as shown in FIG. 4. As a consequence, the average value of the motor driving current does not become zero completely in the vicinity of the inverted portion, causing a level difference portion (i.e., crossover distortion) to occur in the inverted portion. Therefore, the motor cannot be driven smoothly at the portion where the crossover distortion occurs, which leads to disadvantages such as the occurrence of noise or the like.
When the motor is utilized, in particular, as the zoom drive motor of consumer video cameras or the like, then the occurrence of such noise becomes a serious problem because a microphone capsule for collecting sound or the like is usually disposed near the zoom drive motor.